As the Senate takes up an ethics and earmark reform package this week that Democratic leaders have prepared behind closed doors, the chamber’s campaign finance dynamic duo of Sens. John McCain (R-Ariz.) and Russ Feingold (D-Wis.) could make a return performance on the Congressional reform stage — but on opposite sides.

With conservative Republicans bracing for significant changes to the bill passed by the Senate in February, McCain could end up fighting his former ally in an effort to block a significantly weakened bill.

According to sources close to the issue, Majority Leader Harry Reid (D-Nev.) has called on Feingold to help push the bill over the goal line. Reid hopes Feingold’s progressive street credentials and reputation as a reform-minded lawmaker will help keep the left flank from bolting, particularly if Reid and Speaker Nancy Pelosi (D-Calif.) make controversial changes to the bill.

White House hopeful McCain, meanwhile, already has thrown in his lot with Sen. Jim DeMint (R-S.C.), who has attacked the secretive process Reid and Pelosi have used to rewrite the bill and has threatened to filibuster it if its earmark provisions are changed.

Although McCain’s office did not respond to a request for comment, several of his colleagues in the Republican Conference said it appears likely McCain will make a rare appearance in the Senate this week to back reformers. When asked about McCain’s plans, Sen. Lindsey Graham (R-S.C.) said he expected the Arizonan will return to the Senate this week. “That’s my understanding,” Graham said.

Similarly, DeMint said he has discussed the issue with McCain and that while the two had not discussed McCain’s schedule, “I’d expect him to be here. … This was his issue for a long time.”

As for Feingold, spokesmen for the Wisconsin Democrat, Reid and Rules and Administration Chairwoman Dianne Feinstein (D-Calif.) declined to comment on the specific nature of Feingold’s involvement.

“We’ve been consulting with a number of members of the Caucus,” Reid spokesman Jim Manley said. Added Howard Gantman, the majority staff director for the Rules panel: “In terms of finalizing this legislation, we have been discussing it with a number of stakeholders and a number of different offices.”

Although the exact role McCain will play in this week’s ethics showdown remains unclear, it could help DeMint’s prospects for success. While aides to Reid and Senate Minority Leader Mitch McConnell (R-Ky.) last week insisted no weakening of the earmark rules will happen, conservatives approach those assurances with significant skepticism.

One aide noted it is difficult for conservatives to take the word “of a couple of appropriators like Reid and McConnell.” The staffer said DeMint and his allies will reserve judgment until the final deal is released and they are preparing “a bag of shenanigans” to use on the floor to tie up the bill.

Fellow presidential hopeful Sen. Sam Brownback (R-Kan.) joined DeMint and several of their colleagues in signing a July 20 letter to Reid warning against any weakening of the earmark rules as a result of the closed-door talks with Pelosi.

DeMint said last week that he is happy McCain is backing his efforts on earmark reform and warned he will not back down from a fight with Reid.

“I appreciate Sen. McCain and the majority of Republican Senators that support true earmark reform,” DeMint said. “It’s time to end the favor factory in the U.S. Senate and a growing group of reformers won’t sit idly by while some try to keep pork hidden to the public. If Reid refuses to do the right thing and writes a new bill with weak reform, I’ll fight to restore the full earmark rules we agreed to unanimously in January.”

McCain’s presence in the debate could help those efforts to use procedural tactics, aides said. Unlike DeMint, Sen. Tom Coburn (R-Okla.) and other reform-minded conservatives, McCain is a veteran of the Senate and has built strong relationships across the ideological spectrum within the Republican Conference. Additionally, McCain’s national profile certainly would draw added media scrutiny to the issue.

For McCain, fighting what reformists see as watered-down earmark reforms and weak ethics rules also could fit into his efforts to revitalize his flagging presidential campaign.

Aside from Iraq policy, McCain’s last high-profile interjection into legislative action came in June on the comprehensive immigration reform bill. In that case, McCain largely was absent from bipartisan negotiations but then appeared at a Capitol Hill press conference to tout the compromise bill, which fell apart in the Senate and did little to boost his popularity.

Following his poor performance in the fundraising arena and massive layoffs and defections of staff, McCain has decided to try to retake the mantle of the maverick, “straight talk” reformist that helped fuel his 2000 presidential bid.

Ironically, that could bring him into conflict with his old campaign finance partner Feingold.

Long an outcast from the leadership ranks, Feingold has been brought into Reid’s circle in recent months as a key adviser on Iraq. And Feingold and his staff have been working closely with Senate Democratic leadership since last week to help finalize details of the ethics package, according to reform advocates close to the process.

“Their role is to ensure that not only is it done as well as it can be, but also to provide technical expertise,” said Meredith McGehee, policy director at the Campaign Legal Center. “When you’re down to the details, there’s no substitute for that.”

Reform advocates, who had been intimately involved with the drafting of reform provisions this year, said Democratic leaders have been unusually tight-lipped about their plans as they head into the homestretch. But, they said, with Feingold in the room for final talks, they aren’t worried about the outcome.

“We’re encouraged, and we’re hoping it’s a sign they really want to do this right,” Gary Kalman of U.S. PIRG said. “It implies they’re looking for somebody to be a check on loopholes or other problems.”

The thorniest issue remaining as leaders wrapped work on the bill has been harmonizing House and Senate versions of a provision that required lobbyists to disclose for the first time campaign checks they arrange for candidates. The issue provoked sharp resistance in the House from many senior Democratic lawmakers, almost derailing the entire package.

Senate and House negotiators have redrafted the bundling provisions in recent days, shifting the disclosure burden from the lobbyists themselves to the campaigns. The reports, then, will be filed with the Federal Election Commission instead of the Secretary of the Senate and Clerk of the House.

Details of how, exactly, the reporting requirement will be structured were still unclear at press time Friday. An FEC spokeswoman declined to comment.